Thursday, October 14, 2010

Adult Petrels Unaffected by Oil Spill

The European storm petrel (Hydrobates pelagicus) is a pelagic seabird that comes ashore only to breed. Off of the Spanish coast in the bay of Bascay, Aketx islet provides a breeding ground for 2000-7000 petrels yearly. The cliffs on the southeastern corner of the islet provide nesting sites for the European storm petrel as well as shags and yellow-legged gulls.

The tanker “Prestige” carrying 77000 tons of crude oil sank 120 miles off of the coast of Spain in November 2002. Within a month the oil had made its entrance into the bay of Biscay. Over the course of one year clean up crews collected nearly 24000 tons of crude oil as well as 23181 oiled birds. Only 19 oiled European storm petrels were collected which was considered odd. When breeding, European storm petrels are at high risk of being oiled as they feed close to shore and they normally collect their food from water that appears oily.

A study published in the journal Marine Pollution Bulletin describes how long-lived seabirds are able to avoid acute pollution by just skipping breeding. The study, conducted by Jabi Zabala and his research team, focused on the European storm petrel. The study explains that the behavior of the European storm petrels is explained as an example of “fixed investment hypothesis.”

There are two main schools of thought about breeding processes. There is the “flexible investment hypothesis” and the “fixed investment hypothesis.” The “flexible investment hypothesis” is described as when creatures produce many offspring which have low chances of survival. This is very common in short lived species and is also relevant to the oil spill as it lowered the chance of survival for both the offspring and the parents. The “fixed investment hypothesis” is described as when creatures forego reproduction unless the conditions are prime for the survival of the offspring and parents. This behavior is usually exhibited in species with longer life spans.

In the middle of July of 1993 researchers captured, measured, ringed and released European storm petrels on Aketx islet. Every year since, researchers have placed three mist nets in the same location on the island between the hours of 22:30 and 5:30 and repeated the same sampling process of capturing, measuring, ringing and releasing. The number of transients, recaptures and the body conditions were recorded as part of a study on the population. On occasion sampling could not be undertaken due to rough seas. After the sinking of the “Prestige,” sampling was increased but returned to the yearly sampling in 2006.

What the researchers discovered was that the oil spill had no major impact on the adult European storm petrel’s chance of survival every year. Between the years 2003-2005 more transient birds and young birds were captured in the mist nets. The petrels captured during the year after the spill were “suspected to be younger birds with on average poorer body condition.” Zabala stated in the study that from the data there was “64% support for the 'no special effect of the oils spill in petrel survival.’” The results don’t mean that the oil spill had no effect on the entire species, it only refers to how the data demonstrates no major change between the normal year to year survival probability and the survival probability for the year with the oil spill.

The majority of the more experienced breeders and older birds were no where to be found on the islet. It seemed that the older and more experienced petrels decided to skip breeding during the oil spill and for a few years after. The petrels that did remain did not produce many offspring because of the low chance of survival due to the oil spill.

The older and more experienced European storm petrels made a return to Aketx islet in 2006. The average population and reproduction rate soon reached the previous stable levels. By skipping the reproduction while the environment was tainted with oil the older petrels ensured the survival and well being of their own offspring and the wellbeing of their species.